The present invention relates to corrosion resistant zinc powders and a method of manufacturing corrosion resistant zinc powders. More particularly, the present invention relates to high performance zinc alloy powders for use in the anodes of primary and secondary cells and batteries belonging to the zinc-alkaline family, such as zinc-manganese dioxide, silver-zinc, nickel-zinc, zinc-air and zinc-oxygen systems.
Primary and secondary cells experience a loss of capacity on storing because of a self-discharge, parasitic, nonpower-producing reaction of the active electrodes. In particular, the shelf life of a cell employing a zinc electrode is limited by, among other factors, the open circuit and in-use corrosion of the zinc electrode, which causes discharge of the metallic zinc and evolution of hydrogen gas. Thus, in cells employing a zinc electrode, a central issue with respect to cell longevity is the zinc electrode""s resistance to corrosion. Cell longevity is particularly important for zinc electrode batteries that require a long shelf life, for example a year or more, until they are consumed by the electronic application they are intended for.
Zinc anode corrosion is primarily the result of a reaction between the zinc and the aqueous electrolyte, which is commonly an alkaline solution of a Group IA metal hydroxide. One product of this reaction is hydrogen gas, the measurement of which is commonly used to gauge the level of zinc corrosion. The hydrogen gas forms at the cathodic sites of the anode by the decomposition of water. This hydrogen gas is particularly undesirable in sealed batteries where it can lead to bubble formation and excessive pressure build up. Simultaneous to the hydrogen gas generation, active zinc at the anodic sites of the anode oxidizes to zinc hydroxide, zinc oxide and mixtures of zinc hydroxide and zinc oxide. The zinc consumed in this reaction consequently becomes unavailable to produce an electric current, and thereby reduces the electrical capacity of the anode. The self-discharge reactions are as follows:
cathodic sites: 2H2O+2exe2x86x92H2|+2OHxe2x88x92
anodic sites: Zn+2OHxe2x88x92xe2x88x922exe2x86x92Zn (OH)2ZnO+H2O
overall: Zn+2H2Oxe2x86x92H2|+Zn(OH)2ZnO+H2O
To avoid the corrosion of the zinc, a variety of corrosion inhibition techniques have been used. One of the oldest and most effective corrosion inhibition techniques involves the amalgamation of the zinc with mercury. Today, however, environmental policy and laws restrict use of mercury and its compounds as commercial corrosion inhibitors. Other effective techniques of reducing the corrosion reaction include adding corrosion inhibitors to the electrolyte or the zinc and alloying the zinc with an effective amount of a corrosion inhibitor. The challenge with respect to all methods of inhibiting corrosion is to achieve superior corrosion inhibiting performance without significantly sacrificing the zinc anode""s discharge performance within the cell.
Several prior patents relate to the technique of using electrolyte additives to reduce corrosion in zinc anodes. In U.S. Pat. No. 4,112,205, a chloride double salt containing both mercuric ion and quaternary ammonium ion is added to the electrolyte to inhibit corrosion. The corrosion inhibitor provides a relatively continuous source of mercuric ion to the zinc as required to minimize corrosion of unamalgamated zinc as zinc oxide goes into solution and new zinc surfaces are exposed. Again for environmental policy reasons, corrosion inhibitors based on mercury and its salts are not practicable.
U.S. Pat. No. 3,945,849 teaches the use of quaternary ammonium salts as inhibitors for zinc anodes in primary battery cells. The patent teaches that the disclosed quaternary ammonium salts are particularly suitable for use as corrosion inhibitors in Leclanche-type cells wherein the electrolyte employed is ammonium chloride. As such, these corrosion inhibitors are not very effective in alkaline based battery cells in which an alkaline electrolyte comprised of an aqueous solution of a Group IA metal hydroxide is used.
U.S. Pat. No. 4,195,120 teaches alkaline cells containing a predominantly zinc anode and, as-a corrosion inhibitor, a surfactant which is an organic phosphate ester of the ethylene oxide-adduct type. The surfactant is added in such a manner that, directly or upon wetting of the anode with the electrolyte, there is adsorption of the surfactant on the surface of the anode material. The patent, however, teaches that the surfactant should be used in combination with a conventional zinc amalgam powder. This is unacceptable for environmental reasons. Moreover, because the inhibitor works as a surfactant, it tends to interfere with the electrochemical reaction during discharge, which can degrade the electrical performance of the cell to an unacceptable level.
Because mercury is hazardous, research has been conducted on additional anode materials that are able to inhibit the generation of hydrogen in its absence. It has been found that zinc and lead, as well as indium, bismuth and/or gallium, combined in predetermined proportions can produce mercury-free zinc powders that provide effective corrosion resistance. A range of techniques have been investigated for treating zinc with these and other corrosion inhibitor metals to produce corrosion resistant zinc powders. Several commonly employed techniques include thermal atomization, cementation, and electrolytic co-deposition.
In the thermal atomization process, zinc alloy powder is produced using a method in which a predetermined amount of lead or other inhibitor metal is added to high-purity zinc, which is typically produced by an electrolytic method, the entire mixture is then melted to form an alloy. The molten alloy is then atomized through an air jet to form a powder comprised of generally spheroidal or dumbbell-shaped particles of a predetermined size. Typically the resulting zinc powders produced by the thermal atomization process have a bulk density of 2.5-3.5 g/cc, a surface area of 0.1-0.4 m2/g, and a particle size distribution between 0.0075 and 0.8 mm.
Although the thermal atomization technique allows zinc alloy powders to be produced with a wide variety of compositions, thermal atomization has at least two shortcomings over zinc alloy powders produced using the electrolytic co-deposition technique.
First, the morphology of zinc powder produced by the thermal atomization process is less than optimal. In order to achieve high continuous current drain, a large reservoir of active anode material is needed. Due to space and other considerations, this is generally best achieved by incorporating an active anode element having a highly porous morphology and a large surface area of active anodic material. By contrast, in order to achieve high peak power output, studies show that a tight interparticulate packing structure of the active anodic material is advantageous. This has traditionally come at the expense of porosity in known powdered anodes, which can drastically reduce the current capacity of the battery. Therefore, powder morphologies that have large surface areas and yet allow for a tight interparticulate anode structure are desired for battery and cell applications; such morphologies, however, cannot be obtained by the usual thermal processes.
When thermally prepared unamalgamated zinc is compressed sufficiently to form a self-supporting anode, usually with the help of organic binders, the spheroidal particles become well-packed, resulting in a relatively low zinc surface area overall. Lower surface areas limit the utilization potential of the zinc. In a battery application, this results in poor discharge performance. If thermally prepared zinc is used to form an anode with little or no compaction, such as in the case of a gelled anode, the contact area between the solid spherical particles is quite limited, resulting in batteries with low peak power output.
A second shortcoming of thermally produced zinc alloy powder is that it frequently is comprised of a two-phase alloy in which the inhibitor metal phase is not uniformly dispersed throughout the resulting alloy. The characteristically lower homogeneity with respect to inhibitor distribution in the alloy reduces corrosion inhibition and ultimately discharge performance for a given concentration of inhibitor metal.
Another method of treating zinc powder with an inhibitor metal is through cementation. Cementation can be used with thermally prepared zinc or electrolytically prepared zinc. An example of using cementation to treat thermally prepared zinc powder with one or more corrosion inhibitors is described in U.S. Pat. No. 4,084,047. According to this patent, powders of metal oxide inhibitors are thoroughly mixed with zinc and/or zinc oxide powder, typically in a water-slurry. The mixture is then dried and formed into an electrode for a secondary battery using standard pressed powder or paste techniques as are known in the art. The inhibitors taught in this patent are binary combinations of oxides and hydroxides of the Group III and Group IV series of elements, but exclude mercury. In particular, the patent teaches the use of the oxides of Tl, Pb, In, Cd, Sn, and Ga as inhibitors for zinc/zinc oxide electrodes. The resulting electrode is then used in known-in-the-art secondary battery cells employing an alkaline electrolyte. The patent acknowledges that oxides of certain of these inhibitor metals are more soluble in alkaline electrolytes than others. Indeed some are indicated to be virtually insoluble. The more soluble members in the series are said to be excellent corrosion inhibitors of zinc while all of the members of the series are excellent extenders and expanders (compounds that prevent recrystalization and densification of the active zinc material during cell recharging).
Two additional U.S. patents, assigned to the present applicant, describe variations on the cementation technique. U.S. Pat. No. 5,232,798 is directed to a process for inhibiting corrosion in particulate zinc by adding an oxide of an inhibitor metal to an alkaline slurry which includes particulate zinc. The particulate zinc is preferably electrolytically produced and is described as having a density of 0.3-1.4 g/cc and surface area of 0.5-6.0 m2/g. The inhibitor oxide is selected from the oxides of antimony, bismuth, cadmium, indium, gallium, lead, mercury, thallium and/or tin and is preferably added to the slurry in a concentration of 0.05-4.0 parts by weight, based on the weight of the zinc. Similarly, U.S. Pat. No. 5,206,096 discloses a slurry for use in rechargeable metal-air batteries that includes particulate, porous, zinc and an inorganic inhibitor ingredient in an aqueous Group IA metal hydroxide solution. Inhibitor ingredients disclosed in this patent include the inhibitor metal oxides mentioned above in connection with U.S. Pat. No. 5,232,798. The particulate zinc included in the slurry also has properties similar to the particulate zinc disclosed in U.S. Pat. No. 5,232,798.
The problem in general with cementation is that it does not provide uniform corrosion protection throughout the entire discharge process because only the surface of the active zinc particles are treated with the inhibitor metal.
A third method of producing corrosion resistant zinc powders is through the electrolytic co-deposition of the zinc with an appropriate inhibitor metal or metals. A distinct advantage of the electrolytic co-deposition technique for producing corrosion resistant zinc powders is the potential for producing dendritic zinc alloy powders having highly intricate morphologies. The high surface area and compressibility of the dendritic zinc powder particles resulting from certain electroplating techniques can translate into superior discharge characteristics when the resulting powder particles are used to produce zinc anodes. For example, in cell fabrication procedures in which an alkaline slurry containing dendritic zinc particles is extruded or pressed onto an anode current collector, the dendritic zinc particles interlock together and densify. The interlocked zinc particles also bond onto the current collector. This provides a highly conductive zinc/alkaline matrix that is much more effective with respect to discharge capacity and peak power output than an equivalent electrode produced using thermal zinc. The intricate morphology and resulting compressibility of the dendritic zinc makes this improved discharge performance possible.
Furthermore, electrolytically produced zinc alloy powder is more homogeneous in terms of inhibitor distribution because the inhibitor metal is co-deposited throughout the plating process, resulting in a zinc powder product that is intrinsically alloyed throughout with trace inhibitor metals. Corrosion resistant zinc powders produced by physically mixing zinc powder with an inhibitor metal oxide, on the other hand, will have areas of inadequate inhibitor concentration, leading to a reduction in the effectiveness of the inhibitor. Similarly if the zinc is alloyed with the inhibitor metal in a thermal process, microsegregation of an inhibitor metal phase and zinc phase during solidification can result.
A number of patents have investigated the production of electrolytic zinc-inhibitor alloy powders for use in corrosion resistant electrodes. Two such examples are U.S. Pat. No. 5,378,329 and U.S. Pat. No. 5,419,987, both of which have been assigned to the present applicant. These two patents disclose methods of producing electrolytic zinc-inhibitor alloy powder. The methods disclosed in each include the common steps of electrolyzing an admixture comprised of zinc which has been at least partially oxidized to an oxidation product selected from the group consisting of zinc oxide, zinc hydroxide, and zincates; an aqueous solution of a Group IA metal hydroxide or zincate; and an inhibitor metal compound. The inhibitor metal compound being an oxide, hydroxide, carbonate or sulfate of an inhibitor metal such as lead, cadmium, tin, antimony, bismuth, gallium, and indium. Sufficient inhibitor metal compound(s) is dissolved in the aqueous Group IA electrolyte to provide a concentration of 5-1000 ppm of the cation species of the inhibitor metal(s). The electroplating conditions are selected such that a dendritic zinc alloy powder is deposited that contains from 0.001 to 4.0 percent by weight, based on the weight of zinc, of co-deposited inhibitor metal(s).
Alloying zinc with an inhibitor metal using the alkaline plating process described above, however, is limited by the solubility of the inhibitor metal salt in the electrolyte. Inhibitor metals having salts with low solubility will naturally alloy with zinc in lower concentrations. However, because corrosion retardation is a function of inhibitor metal concentration, some otherwise effective corrosion inhibitors cannot be alloyed with zinc in sufficient concentrations to obtain maximum protection using the electrowinning method described above. For example, while some inhibitor compounds like the oxides and hydroxides of lead, gallium and tin dissolve well in concentrated alkaline solutions, others such as the oxides and hydroxides of indium and bismuth, do not. In particular, both In2O3 and Bi2O3 have a maximum solubility of less than about 10 ppm in aqueous electrolytes containing KOH; the maximum solubility of In and Bi in concentrated KOH solutions being approximately 7 ppm and 5 ppm, respectively. Consequently, alloying zinc with these metals using known alkaline based electrolytic co-deposition techniques has not been able to fully exploit their potential as corrosion inhibitors for zinc anode applications. For example the maximum practicable concentration of In and Bi that can be electrolytically co-deposited with zinc in an electrowinning process of the type described above is approximately 200 ppm and 200 ppm, respectively.
Because sparingly soluble inhibitor metal compounds, such as In2O3 and Bi2O3, tend to slowly dissolve in the electrolyte, it is also difficult to maintain the concentration of the corresponding inhibitor metal in the electrolyte during the electrodeposition process when deposition rates practicable for commercial application are employed. As a result, it is currently infeasible to co-deposit a uniform concentration of a sparingly soluble inhibitor metal in a commercial electrowinning process.
A need exists, therefore, for an improved electrolytic method of producing dendritic zinc-inhibitor metal alloy powders that include inhibitor metals, the salts of which are only sparingly soluble in aqueous alkaline solutions. In particular, a need exists for a method of electrolytically producing zinc-inhibitor metal alloy powders that includes an alkaline electrodeposition process in which inhibitor metals that are only sparingly soluble in alkaline electrolytes can be co-deposited in concentrations that are not otherwise possible using current zincate electrowinning processes. Furthermore, to obtain superior performance in battery and cell applications, the resulting zinc-inhibitor metal alloy powder should have a high surface area and a dendritic structure with an intricate morphology. A need particularly exists for an electrolytic method of producing mercury-free zinc-inhibitor metal alloy powders that include indium in a concentration greater than 200 ppm or bismuth in a concentration greater than 200 ppm and that have a dendritic structure with a large surface area and an intricate morphology.
According to the present invention, there is now provided an improved method of producing mercury-free dendritic zinc-inhibitor metal alloy powders that include inhibitor metals, the salts of which are only sparingly soluble in aqueous alkaline solutions. The present invention also provides a method of electrolytically producing mercury-free zinc-inhibitor metal alloy powders that includes an alkaline electrodeposition process in which inhibitor metals that are only sparingly soluble in alkaline electrolytes can be co-deposited in concentrations that are not otherwise possible using currently known zincate electrowinning processes.
For purposes of this invention, an inhibitor metal salt, such as the oxide, hydroxide, carbonate, or sulfate of the inhibitor metal, is xe2x80x9csparingly solublexe2x80x9d in an alkaline electrolyte if less than about 10 ppm of the inhibitor metal salt is soluble in the alkaline electrolyte.
The method of producing mercury-free corrosion resistant dendritic zinc alloy powder according to one embodiment of the present invention comprises the steps of: a) preparing an electrolytic cell containing an aqueous alkaline electrolyte with a preselected concentration of dissolved zinc cations in solution, a non-zinc adherent cathode, a first anode, and a second anode, wherein the second anode comprises a first inhibitor metal, the salts of which are only sparingly soluble in the alkaline electrolyte; b) applying a first voltage between the first anode and cathode to establish a desired cathode current density and the deposition of dendritic zinc on the cathode; c) applying a second voltage between the second anode and cathode to establish a desired current density at the second anode and the simultaneous co-deposition of a desired concentration of the first inhibitor metal in the dendritic zinc; d) periodically removing the deposited zinc alloy from the cathode; and e) homogenizing the removed zinc alloy into a plurality of dendritic zinc alloy particles.
The resulting dendritic zinc alloy powder particles preferably have an average surface area between 0.5 and 1.8 m2/g, a bulk density of between 0.5 and 1.3 g/cc, and an average particle size in the range of about 50 to 200 microns.
Preferably the aqueous alkaline electrolyte comprises an aqueous Group IA metal hydroxide solution, such as a solution containing potassium hydroxide, sodium hydroxide, or lithium hydroxide. As those skilled in the art will recognize, there are a number of potential sources for the dissolved zinc cations in the electrolyte. These include, by-way of example, zinc that has been at least partially oxidized to an oxidation product of zinc oxide, zinc hydroxide, or zincate.
Examples of non-zinc adherent cathodes suitable for practicing the present invention include stainless steel, magnesium, titanium, and vitreous carbon. Other suitable materials, however, may also be employed in the present invention and will be immediately apparent to those skilled in the art from reviewing the present disclosure.
The first anode is preferably a major anode and can be of the inert type, for example a nickel anode, optionally coated with an electrocatalyst having a low oxygen evolution over-voltage, or it can be of the dissolving reactive type, for example a zinc or zinc alloy anode. The second anode is preferably a minor anode comprised of an inhibitor metal, the salts of which are only sparingly soluble in the alkaline electrolyte. In the preferred embodiments of the invention, the second anode is preferably comprised of indium or bismuth, but, in practice, other inhibitor metals that are only sparingly soluble in alkaline electrolytes can also be used. Further, it will be apparent to those skilled in the art after reviewing the present disclosure that multiple major and minor anodes can be used in practicing the present invention to obtain dendritic zinc alloy particles of the desired composition and morphology. For example a combination of inert and reactive major anodes can be used. Similarly, one minor anode can be made out of indium and another out of bismuth. Alternatively, when multiple minor anodes are employed, other inhibitor metals that are alloyable with zinc can be used to form one or more of the minor anodes. However, at least one of the anodes in the electrolytic cell must comprise an inhibitor metal, the salts of which are only sparingly soluble in the alkaline electrolyte.
The aqueous alkaline electrolyte may also include cations of at least one inhibitor metal selected from the group consisting of lead, gallium, and tin. The cation species of these additional inhibitor metals are preferably provided by dissolving the oxide, hydroxide, carbonate or sulfate of the inhibitor metal(s) in the aqueous Group IA metal hydroxide electrolyte. Other compounds of the inhibitor metal may be used provided they do not introduce contaminant species or ions into the bath. When cations of one or more of these additional inhibitor metals are present in the electrolyte, they will co-deposit with the dendritic zinc as it is electroplated on the cathode and, as a result, they will be intrinsically alloyed therein.
As will be evident from the foregoing, a variety of zinc-inhibitor metal alloys can be produced using the method according the present invention. However, because at least one of the anodes included in the electrolytic cell will always comprise an inhibitor metal that is only sparingly soluble in the alkaline electrolyte, at least one sparingly soluble inhibitor metal will always be included in the electrodeposited zinc alloy.
The concentration of the sparingly soluble inhibitor metal in the final zinc alloy is directly controllable by varying the voltage between the second anode and cathode and thus the current density at the second anode. As a result, the dissolution reaction at the second anode can be driven forward by increasing the current density at the second anode. This makes it possible to co-deposit sparingly soluble inhibitor metals, such as bismuth and indium, in concentrations significantly greater than was previously possible when these inhibitor metals were co-deposited directly from an aqueous alkaline electrolyte in a traditional electrowinning process employing a cell with a standard inert, or even a soluble zinc, anode. The present invention also makes it commercially feasible for the first time to prepare dendritic zinc-inhibitor alloy powders that include effective concentrations of sparingly soluble inhibitor metals in an alkaline electrodeposition process.
If the concentration of indium and/or bismuth that is co-deposited in the method according to the present invention is within the range of 75 to 3500 ppm and 50 to 2400 ppm, respectively, corrosion inhibition can be obtained. Preferably, however, the concentration of indium and/or bismuth that is co-deposited is within the range of 250 to 3000 ppm and 100 to 2000 ppm, respectively. More preferably, the concentration of indium and/or bismuth that is co-deposited is within the range of 500 to 3000 ppm and 250 to 2000 ppm, respectively. And even more preferably, the concentration of indium and/or bismuth that is co-deposited is within the range of 1000 to 2600 ppm and 1000 to 1800 ppm, respectively.
Effective and preferred ranges of lead and gallium that can be co-deposited in the zinc alloy are provided in the table below:
Unless otherwise noted, it is to be understood that the various concentrations of inhibitor metals expressed in terms of ppm and referenced throughout this application as being present in the zinc alloy are based on the weight of zinc and not the combined weight of zinc and the inhibitor metal(s).
Particularly preferred mercury-free electrolytic zinc alloy powders produced in accordance with the present invention include indium in a concentration greater than 200 ppm or bismuth in a concentration greater than 200 ppm and have a dendritic structure with a large surface area and an intricate morphology. The preferred zinc alloy powders according to the present invention include bismuth or indium in concentrations greater than 200 ppm because previously this was the maximum practicable concentration at which indium or bismuth could be electrolytically co-deposited with zinc to form dendritic zinc alloy powder particles.
Thus, according to a preferred embodiment of the present invention, a mercury-free corrosion resistant zinc alloy powder is provided that is comprised of a plurality of electrolytically prepared dendritic zinc alloy particles having a surface area between 0.5 and 1.8 m2/g and a bulk density of between 0.5 and 1.3 g/cc. The zinc alloy according to this embodiment comprises zinc as a major alloying element and bismuth as a minor alloying element in a concentration of between about 250 and 2400 ppm, the bismuth being intrinsically alloyed throughout each of the dendritic zinc alloy particles. In addition, the zinc alloy preferably further comprises at least one additional inhibitor metal selected from the following group in the specified concentration: lead in a concentration of about 20 to 2500 ppm, indium in a concentration of about 75 to 3500 ppm, and gallium in a concentration of 10 to 100 ppm. For improved performance in battery or cell applications, it is desirable for the zinc alloy particles to have an average particle size in the range of about 50 to 200 xcexcm.
According to another preferred embodiment of the invention, a mercury-free corrosion resistant zinc alloy powder is provided that is comprised of a plurality of electrolytically prepared dendritic zinc alloy particles having a surface area between 0.5 and 1.8 m2/g and a bulk density of between 0.5 and 1.3 g/cc. The zinc alloy according to this embodiment of the invention comprises zinc as a major alloying element and indium as a minor alloying element in a concentration of between about 250 and 3500 ppm, the indium being intrinsically alloyed throughout each of the dendritic zinc alloy particles. Preferably the zinc alloy further comprises at least one additional inhibitor metal selected from the following group in the specified concentration: lead in a concentration of about 20 to 2500 ppm, bismuth in a concentration of about 50 to 2400 ppm, and gallium in a concentration of 10 to 100 ppm. For improved battery or cell performance, it is also preferable for the zinc alloy particles to have an average particle size in the range of about 50 to 200 xcexcm.
Further objects and advantages of the present invention will be better understood from the following description considered in connection with the accompanying drawings in which the preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings and examples are for the purposes of illustration and description only and are not intended as a definition of the limits of the invention.